(1) Field of the Invention
The present invention relates to a switching power supply apparatus, and particularly relates to a switching power supply apparatus which inhibits change in output voltage when there is a sudden change in load.
(2) Description of the Related Art
A conventional switching power supply apparatus is described with reference to the drawings.
FIG. 13 is a circuit diagram showing a circuit configuration of a switching power supply apparatus of which an input side and an output side are electrically insulated. The switching power supply apparatus shown in the figure includes, for example, a switching device 104 made up of a power MOSFET and so on and a control circuit 130 which controls the switching device 104.
In the switching power supply apparatus shown in FIG. 13, for example, an alternating current (AC) which is inputted to a main input terminal from a commercial power source is rectified by a rectifier 101 configured with a diode bridge and so on. The AC is subsequently smoothed by an input capacitor 102 into a direct-current (DC) voltage VIN, and then is inputted to a transformer 103 for power conversion. The transformer 103 includes a primary winding 103a, an auxiliary winding 103b, and a secondary winding 103c, and the smoothed DC voltage VIN is inputted to the primary winding 103a. 
The DC voltage VIN inputted to the primary winding 103a of the transformer 103 is controlled by the switching device 104. At this time, as a result of the switching operation of the switching device 104, an electromotive force is generated by electromagnetic induction in the secondary winding 103c of the transformer 103.
The current generated in the secondary winding 103c is rectified and smoothed by a diode 110 and an output capacitor 111 which are connected to the secondary winding 103c, so as to be supplied to a load 112 as the DC of an output voltage VO.
An electromotive force is also generated in the auxiliary winding 103b of the transformer 103, and the AC output from the auxiliary winding 103b is rectified and smoothed by a power voltage generation circuit 120 made up of a diode 121 and a capacitor 122, so that a control terminal voltage Vcc is generated.
The control circuit 130 driven by the control terminal voltage Vcc outputs a switching signal to a gate of the switching device 104. Here, the control terminal voltage Vcc is compared to the output voltage VO supplied to the load 112 from the secondary winding 103c of the transformer 103, and is also used as a return signal for stabilizing the output voltage VO.
The control circuit 130 includes: an oscillator 131 which outputs a clock signal CLOCK and a maximum duty cycle signal MAXDC for the switching signal to be outputted to the switching device 104; an error amplifier 132 which outputs an error voltage signal VEAO depending on a difference between the control terminal voltage Vcc and a reference voltage; a device current detection circuit 133 which detects a switching device current ID flowing in the switching device 104, so as to output a device current detection signal VCL; a comparator 134 which compares the error voltage signal VEAO and the device current detection signal VCL, so as to output a result of the comparison; and a switching signal control circuit 135 which controls an output of the switching signal based on the result of the comparison.
The switching signal control circuit 135 includes an RS flip-flop circuit 136 which receives, at a set terminal, the clock signal CLOCK from the oscillator 131 and receives, at a reset terminal, a signal output from the comparator 24; a NAND circuit 137 which receives, at an input terminal, a maximum duty cycle signal MAXDC from an oscillator 131 and receives, at another input terminal, a signal output from the RS flip-flop circuit 136; and a gate driver 138 which receives a signal output from the NAND circuit 137, so as to output a switching signal by performing inverting amplification on the signal.
The following will describe an operation of the conventional switching power supply apparatus configured as above.
In FIG. 13, immediately after the switching power supply apparatus is started, when an AC from a commercial power source is inputted to the rectifier 101, the inputted AC is rectified and smoothed by the rectifier 101 and the input capacitor 102 to be converted into a DC voltage VIN, and the converted DC voltage VIN is applied to the primary winding 103a of the transformer 103. At this time, the DC voltage VIN is applied to an internal current supply circuit 139 included in the control circuit 130, and the capacitor 122 of the power voltage generation circuit 120 is charged with a current supply from the internal current supply circuit 139.
After this, in the power voltage generation circuit 120, the control circuit 130 starts operating when the control terminal voltage Vcc reaches the start-up voltage of the control circuit 130. This causes the switching device 104 to start controlling the switching operation and also causes a start-and-stop circuit 140 to stop the current supply from the internal current supply circuit 139.
The control circuit 130 controls the switching operation performed by the switching device 104, based on the control terminal voltage Vcc so as to stabilize the output voltage VO to the load 112 at a predetermined voltage. Specifically, the output voltage VO to the load 112 and the control terminal voltage Vcc are applied in accordance with a ratio between the auxiliary winding 103b and the secondary winding 103c of the transformer 103. Furthermore, the comparator 134 compares the error voltage signal VEAO from the error amplifier 132 and the device current detection signal VCL from the device current detection circuit 133, so as to provide a high-level output signal to the reset terminal of the RS flip-flop circuit 136 when the error voltage signal VEAO and the device current detection signal VCL become equal to each other. This controls the switching operation so that the output voltage VO returns to the predetermined voltage even when the output voltage VO changes.
FIG. 14 is a timing chart showing an operation of the conventional switching power supply apparatus having such a configuration.
As the figure shows, at the time of change in load, when a load supply current IO decreases due to a decrease in the amount of current supplied to the load 112, an output voltage VO slightly increases. In response to this, the control terminal voltage Vcc of the power voltage generation circuit 120 increases on a return side, and the error voltage signal VEAO from the error amplifier 132 decreases.
When the error voltage signal VEAO and the device current detection signal VCL become equal to each other, a reset signal is outputted from the comparator 134 to the reset terminal of the RS flip-flop circuit 136: thus, at the time of decrease in the load current, due to decrease in error voltage signal VEAO, the NAND circuit 137 outputs a signal for turning off the switching device 104 with timing earlier than in a steady-load period. As a result, this shortens the time for which the switching device 104 is on during the switching operation, causing a decrease in the switching device current ID flowing in the switching device 104.
Thus, the control circuit 130 in the conventional switching power supply apparatus stabilizes the output voltage VO in response to a change in the load supply current IO supplied to the load 112, by controlling the duty cycle of the switching device 104 according to the control terminal voltage Vcc proportional to the output voltage VO. Specifically, the control circuit 130 performs control to make the peak of the switching device current ID shorter by shortening the duty cycle of the switching device 104 as the control terminal voltage Vcc is larger. Patent Reference 1 (Japanese Patent No. 3434788), for example, discloses the technique related to this control.